Figure out how many sts per inch you get with the needle you're going to use and multiply that by how many inches around your head measures less 2" and that's your cast on number. You may have to adjust by 1 or 2 depending on if you use a ribbing for your edge. Here's a basic hat pattern you can use for other measurements, and for a slouch you'd probably want to make it a couple inches longer before doing the decreases. At that point you can try it on to see just how much.

I'd suggest going to Ravelry.com. You have to sign up but it's free to join. Input slouchy beanie in the pattern search box. You'll see a bunch of pictures for patterns come up. On the left, you can narrow your results. Only knitting, only free patterns, only using bulky yarn, for a child or adult, input the needle size you want to use. This will give you patterns with your conditions. Do follow the needle size recommendation on the yarn wrapper. It's a pain to knit bulky yarn with needles that are too small.